In order to improve the performance of an LTE-A (Long Term Evolution-Advanced) system and increase the capacity of a PDCCH (Physical Downlink Control Channel), an E-PDCCH (Enhanced PDCCH) is introduced into the Rel-11. The present standard has determined that the E-PDCCH supports two modes of beam forming transmission and diversity transmission, which are applied to different scenarios. For the diversity transmission mode, the specific transmission solution has not been proposed.
In an LTE system, the PDCCH is transmitted in each radio subframe and forms a TDM (Time Division Multiple) relation together with a PDSCH (Physical Downlink Shared Channel). As shown in FIG. 1, the PDCCH is transmitted through the first N OFDM (Orthogonal Frequency Division Multiplex) symbols of one downlink subframe, wherein the value of N may be 1, 2, 3 or 4, and N=4 is merely permitted in a system with a system bandwidth of 1.4 MHz. A control area for transmitting the PDCCH in the LTE system is formed by logically divided CCEs (Control Channel Elements), wherein the mapping from the CCEs to REs (Resource Elements) adopts a complete interleaving manner. The transmission of DCI (Downlink Control Information) is also based on the CCEs serving as units, one DCI for one UE may be transmitted in N continuous CCEs, and the value of N in the LTE system may be 1, 2, 4 or 8 and is referred to as a CCE aggregation level. The UE (User Equipment) performs PDCCH blind decoding in the control area to search whether the PDCCH transmitted for the UE exists, the blind decoding means decoding attempt is performed on the DCI of different formats and the different CCE aggregation level by using an RNTI (radio network temporary identifier) of the UE, and if the decoding is correct, the DCI for the UE is received. The LTE UE in a non-DRX (Discontinuous Reception) state needs to perform blind decoding on the control area in each downlink subframe to search for the PDCCH.
Due to the introduction of technologies of MU-MIMO (Multi-user Multiple Input Multiple Output), COMP (Cooperative Multiple Points), carrier aggregation and the like and configurations of RRHs (Remote Radio Heads) with the same cell ID, 8 antennas and the like, the capacity and transmission efficiency of the PDSCH of the LTE-A (Long Term Evolution-Advanced) system are greatly improved; and compared with an early LTE release (such as Rel-8/9), the PDCCH of the LTE-A system is not improved as a result of the new technologies.
On the one hand, the application of the new technologies enables the PDSCH to simultaneously provide data transmission for more users, thus greatly increasing the requirement for the capacity of the PDCCH; and on the other hand, the new technologies of DM-RS (Demodulation Reference Symbol) used in the PDSCH, R-PDCCH (Relay PDCCH) used in Relay backhaul and the like provide followable technology and experience for enhancement of the PDCCH.
To solve the problem that the capacity of the downlink control channel is limited and improve the transmission efficiency of the downlink control information, one solution is to reserve the original PDCCH field and transmit an enhanced PDCCH (namely E-PDCCH) in the PDSCH field of the downlink subframe, as shown in FIG. 2. The original PDCCH field still adopts the existing transmitting method and receiving method and uses the original PDCCH resources, for example, adopts transmission diversity during transmitting, performs blind decoding on the DCI in a common search space and a UE-specific search space based on a CRS (Cell-specific Reference Signal) during receiving, and occupies the first N OFDM symbols for transmitting, wherein the value of N may be 1, 2, 3 or 4, N=4 is merely permitted in the system with the system bandwidth of 1.4 MHz, and this PDCCH field is referred to as a legacy PDCCH (original PDCCH or legacy PDCCH) field.
The E-PDCCH field may use more advanced transmitting and receiving methods, for example, adopts pre-coding during transmitting, performs decoding based on DM-RS during receiving, occupies time frequency resources for transmitting other than the legacy PDCCH field, uses a part of resources of the original PDSCH and is multiplexed with the PDSCH in a frequency division manner, and this PDCCH field is referred to as the Enhanced PDCCH (E-PDCCH) field. This solution of multiplexing the E-PDCCH and the PDSCH in the frequency division manner is referred to as an FDM E-PDCCH (frequency division multiplexing E-PDCCH).
The present standard has determined that the E-PDCCH supports two modes of beam forming transmission and diversity transmission, which are applied to different scenarios. Generally, the beam forming transmission mode is mostly used in a scenario that a base station can obtain relatively precise channel information fed back by a UE and the change of neighbor cell interference is not very severe along with the change of a subframe, and in this case, the base station selects continuous frequency resources with relatively high quality according to the CSI fed back by the UE to transmit the E-PDCCH for the UE and performs beam forming processing to improve the transmission performance. Under the condition that the channel information cannot be accurately obtained or the change of the neighbor cell interference is severe along with the change of the subframe and unpredictable, the E-PDCCH needs to be transmitted in the frequency diversity manner, namely transmitted by using frequency resources with discontinuous frequencies.
FIG. 3A shows an example of transmitting the E-PDCCH by using the continuous frequency resources, FIG. 3B shows an example of transmitting the E-PDCCH by using the discontinuous frequency resources, the transmission of one DCI in each example occupies the resources in four PRB pairs; and several basic concepts will be illustrated below.
A PRB (Physical Resource Block) is a resource unit consisting of one slot in a time domain and one RB in a frequency domain; wherein, the one slot consists of 7 continuous OFDM symbols under the normal CP condition or 6 continuous OFDM symbols under the extended CP condition, and the following described CP takes a normal CP as an example; and the one RB consists of 12 continuous subcarriers in the frequency domain.
A PRB pair is a resource unit consisting of two slots in one subframe in the time frequency and one RB in the frequency domain.
A legacy PDCCH, namely a PDCCH defined in the LTE Rel-8/9/10, is transmitted in a carrier to which an LTE Rel-8/9/10 UE can access, occupies the whole system bandwidth in the frequency domain, and occupies the first 1, 2, 3 or 4 continuous OFDM symbols of the first slot in a downlink subframe in the time domain; and in an added type of carrier to which an LTE Rel-8/9/10 UE cannot access, the legacy PDCCH may not be transmitted.